The present invention relates to a display device which utilizes an emission of electrons into a vacuum space, which is formed between a face substrate and a back substrate to produce a display; and, more particularly, the invention relates to a display device of the type described which exhibits excellent characteristics in emitting electrons from an electron source.
As a display device which exhibits a high brightness and high definition, color cathode ray tubes have been popularly used conventionally. However, along with the recent request for the provision of higher quality images in information processing equipment or television broadcasting, the demand for planar displays (panel displays) which are light in weight and require a small space, while exhibiting a high brightness and high definition, has been increasing.
As typical examples, liquid crystal display devices, plasma display devices and the like have been put into practice. Further, as display devices which can realize higher brightness, it is expected that various kinds of panel-type display devices, including a display device, which utilizes an emission of electrons from electron sources into a vacuum and is referred to as an electron emission type display device or a field emission type display device, and an organic EL display, which is characterized by low power consumption, will be commercialized.
Among such panel type display devices, such as the above-mentioned field emission type display device, a display device having an electron emission structure, which was proposed by C. A. Spindt et al, a display device having an electron emission structure of a metal-insulator-metal (MIM) type, a display device having an electron emission structure which utilizes an electron emission phenomenon based on a quantum theory tunneling effect (also referred to as “surface conduction type electron source), and a display device which utilizes an electron emission phenomenon having a diamond film, a graphite film and carbon nanotubes and the like, have been known.
Among these panel type display devices, the field emission type display device is formed by laminating and sealing a front panel, which has an anode electrode and a fluorescent material layer formed on an inner surface thereof, and a back panel, which has electron emission type cathodes and grid electrodes, which constitute a control electrode, formed on an inner surface thereof, so that a distance of not less than 0.5 mm, for example, is formed therebetween, whereby a sealed space is formed between both panels and the sealed space is evacuated to a pressure lower than the ambient atmospheric pressure or to a vacuum.
Recently, the use of carbon nanotubes (CNT) as a field emission type electron source, which constitutes the cathodes of this type of planar display, has been studied. Carbon nanotubes are extremely thin needle-like carbon compound elements (strictly speaking, a so-called graphene sheet in which carbon atoms are coupled in a hexagonal shape is formed in a cylindrical shape). A carbon nanotube assembly, which is formed by collecting a large number of carbon nanotubes, is fixed to a cathode electrode. By applying an electric field to the cathode electrode having the carbon nanotubes, it is possible to emit electrons with a high density from the carbon nanotubes with a high efficiency, whereby it is possible to constitute a flat panel display which is capable of displaying various images of high brightness by exciting a phosphor with these electrons.
FIG. 13 is a schematic diagram showing the basic structure of a field emission type display device. CNT denotes the carbon nanotubes formed on a cathode (cathode electrode) K, A indicates an anode (anode electrode), and a phosphor PH is formed on an inner surface of the anode A. A grid electrode G, which controls the emission of electrons, is formed in the vicinity of the cathode K, and a voltage Vs is applied between the cathode K and the grid electrode G so that electrons are emitted from the carbon nanotubes CNT. By applying a high voltage Eb between the cathode K and the anode A, the electrons e emitted from the carbon nanotubes CNT are accelerated and the phosphor PH is excited whereby light L having a color which is dependent on the composition of the phosphor PH is irradiated. Then, by controlling the quantity of electrons which are emitted in response to the modulation voltage Vs that is supplied to the grid electrode G formed in the vicinity of the cathode K, for example, the brightness of the light L can be controlled.
FIG. 14 is a schematic cross-sectional view showing a constitutional example of the field emission type display. In this field emission type display (FED), a back substrate 1 which is formed of a glass plate and a face substrate 2 which is also formed of a glass plate are laminated to each other by way of a frame-like support body 3 which is interposed between both substrates 1, 2. The support plate 3 has a height of approximately 1 mm, for example, and surrounds a display region so as to maintain a given distance between both substrates 1, 2. Further, the inside hermetic space between the substrates is evacuated and sealed. Cathode lines 13, insulation layers 14 and grid electrodes 15 are formed on an inner surface of the back substrate 1, while anode electrodes 11 and phosphors 12 are formed on an inner surface of the face substrate 2. Carbon nanotubes of electron sources which are not shown in the drawing are provided to the cathode lines 13.
FIG. 15 is a schematic plan view as seen from the back substrate 1 side of the field emission type display shown in FIG. 14. In the inside of the effective display region AR on the inner surface of the face substrate 2, phosphors R, G, B of three colors are arranged. In this example, respective pixels are defined by partitions 16. In a monochromic display, all phosphors are formed in the same color.
With respect to the above-mentioned display which uses carbon nanotubes, various publications, such as non-patent literature 1 (“Large Size FED with Carbon Nanotube Emitter” Sashiro Uemura et al., SID 02 DIGEST (2002), pp. 1132-1135), non-patent literature 2 (Fully sealed, high-brightness carbon-nanotube field-emission display”., W. B. Choi et al., Appl. phys. Lett., VOL. 75, NO. 20, (1999), pp. 3129-3131) and the like are known. A field emission type display disclosed in these publications is configured such that a carbon nanotube paste, which is obtained by forming carbon nanotube powder into a paste, or a carbon nanotube-metal mixture paste, which is formed by mixing carbon nanotube powder and metal powder, is printed on a glass substrate, and gate electrodes which constitute pull-out electrodes (or control electrodes) and a fluorescent surface which emits light upon incidence of the pulled-out light are arranged on an upper surface of the printed paste.
Further, as examples of cathodes which constitute electron emitting portions in this type of panel display, a technique in which the electron emitting portions are constituted of carbon nanotubes formed of cylindrical graphite layers is disclosed in patent literature 1 (Japanese Unexamined Patent Publication Hei11(1999)-162383. Further, patent literature 2 (Japanese Unexamined Patent Publication 2000-36243) discloses a method of forming an electron emission portion in which a paste which is formed by mixing bundles, each of which is a mass of carbon nanotubes into a tacky solution having conductivity is formed into a pattern, and laser beams are irradiated to the pattern thus making the carbon nanotubes emit electrons in a state in which the carbon nanotubes are projected from a surface of the pattern.
Further, patent literature 3 (Japanese Unexamined Patent Publication 2000-90809) discloses a technique in which field emission cathodes are formed by causing a bundle of carbon nanotubes to adhere to a substrate using a conductive resin. Still further, patent literature 4 (Japanese Unexamined Patent Publication 2000-251783) discloses an example in which a resistance layer formed of a ruthenium oxide mixture film or an a-Si thin film is applied to a cathode electrode formed of a strip-like conductor, and an emitter made of a field emission material, such as carbon nanotubes, is formed on the resistance layer. Further, patent literature 5 (Japanese Unexamined Patent Publication 2001-283716), patent literature 6 (Japanese Unexamined Patent Publication 2002-157951) and the like disclose a technique in which a portion of carbon nanotubes is embedded into a metal plating layer formed on a support substrate and projecting portions are used as an emitter.